The invention relates to a process of purifying a fluid medium with a predetermined minimum radiation dose of ultraviolet radiation wherein, at a given location a number n, less than six, of flow paths for said medium are provided in juxtaposition to each other and the incident ultraviolet radiation is first introduced into one of said paths and permitted to escape from said one path into other of said paths. The invention also relates to an apparatus for purifying a fluid medium at a predetermined minimum radiation dose of ultraviolet radiation comprising a flow reactor defining an irradiation volume having two sides and through which volume said medium flows, at least one ultraviolet radiation source positioned to introduce ultraviolet radiation into the medium into said volume at least at one of said sides, and comprising further at a given location within said irradiation volume first means forming windows transparent to ultraviolet radiation and spaced from each other and second means defining one side of said irradiation volume, a total number n, less than six, of successive irradiation chambers and windows being provided through which the radiation will pass from one irradiation chamber to the next.
Such processes and apparatus are described in my co-pending application Ser. No. 923,710. The disclosure of my co-pending application Ser. No. 923,710 filed July 11, 1978, now U.S. Pat. No. 4,255,383, and entitled Multichamber Photoreactor is incorporated herein by reference. The particular advantages gained by combining the effects of the UV radiation on the medium in the different irradiation flow paths are discussed therein with reference to presently used known single chamber photoreactors and with reference to known multichamber photoreactors of different configuration as well as with reference to the significant increase in efficiency achieved by limiting the absorption by the medium in the flow path immediately adjacent to the radiation source to not more than 50 percent and the total absorption in all the flow paths to values increasing with the number of flow paths, for instance to 75 percent, if two flow paths are present.
The present invention is based on the recognition that at a predetermined minimum dose the efficiency in terms of flow-dose rate achievable in a single chamber photoreactor is determined by the depth of the medium therein, said flow-dose rate depending on the volume of the irradiation chamber and on the effective irradiation intensity. If the flow-dose rate at the required minimum dose M (in milliwatt seconds per cm.sup.2) results from the rate of flow Q (in m.sup.3 /h) and from the irradiation intensity (in milliwatts per cm.sup.2), the flow-dose rate Q-M is a function of the volume V of the medium and the radiation dose E EQU Q-M=f(V, E)
Therein, Q-M is the flow-dose rate at the required minimum dose M, V is the volume of the irradiation chamber, and E is the radiation intensity. Calculations will have to be based on the minimum radiation intensity effective within the volume V of the reactor chamber and not on the intensity of the radiation entering the reactor. The volume of the reactor chamber increases and the effective radiation intensity decreases with increasing depth d EQU Q-M=V(d).multidot.E(d)
In accordance with the rules of differential calculus a maximum for the flow-dose rate will result therefrom ##EQU1## The volume of the reactor chamber is EQU V(d)=d.multidot.F
F being the face through which the radiation enters. The effective radiation intensity will be that radiation intensity which remains after the incident radiation has passed through a depth d of the medium to be irradiated. EQU E.sub.d =E.sub.o .multidot.10.sup.-.epsilon..multidot.d bzw. E.sub.d =E.sub.o .multidot.e.sup.-kd
For a single chamber photoreactor traversed by parallel radiation a maximum in the flow-dose rate will result therefrom for EQU -d.multidot.F.multidot.E.sub.o .multidot.k.multidot.e.sup.-kd =-F.multidot.E.sub.o .multidot.e.sup.-kd
i.e. d.multidot.k=1 and d.multidot.ln T=1, respectively, PA1 and .epsilon..multidot.d=log e and d.multidot.log T=log e, respectively.
Therein, k is the extinction coefficient related to the base e of the medium to be irradiated at a wavelength of 254 nm which forms the basis for the entire wavelength range of the effective UV radiation; T is the UV transmission of the medium to be irradiated as measured in a 1 cm cuvette, also at a wavelength of 254 nm; .epsilon. is the specific decadic extinction coefficient. The product .epsilon..multidot.d usually is termed extinction. In this context "absorption" is the portion A of the incident radiation intensity E.sub.o absorbed in the depth d of the medium and resulting from A=E.sub.o -E.sub.d according to the Lambert Beer Law of Absorption and which may also be expressed in percent of the incident radiation intensity E.sub.o.
In the case of a single chamber photoreactor traversed radially by radiation directed outwardly from the interior, the irradiation chamber being arranged coaxially with respect to an envelope tube having the radius r.sub.i and surrounding the radiation source, there is EQU V=(r.sub.i +d).sup.2 -.pi.r.sub.i.sup.2 =.pi..multidot.d (2r.sub.i +d)
and ##EQU2## respectively. Therefrom is obtained ##EQU3## While complete differentiation of the foregoing expression will result in a complex expression, an approximative consideration shows that the quotient will only have small influence on the slope of the flow-dose rate function and may be approximately accounted for within the usual values of r.sub.i in the exponent. Thus EQU V(d).multidot.E(d)=d.multidot.F.multidot.E.sub.o .multidot.e.sup.-Kd
The maximum of the flow-dose rate will be at dK=1. The optimized annular photoreactor and the optimized photoreactor traversed by parallel radiation are interrelated by K=k.multidot..alpha. so that the maximum flow-dose rate for the first one will exist at dK=1/.alpha., which empirically is found to be dk=0.826.
The aforementioned single chamber photoreactors thus have a maximum in the flow-dose rate at a depth d at which the intensity of the effective radiation E.sub.o incident into the reactor has decreased to e.sup.-1, i.e. to 36.8 percent, in the case of parallel radiation and to e.sup.-0.826, i.e. to 43.7 percent, in the case of radially directed radiation. With smaller depths the flow-dose rate will deteriorate because of imperfect utilization of the available radiation intensity. With higher depths the flow-dose rate will deteriorate because of the increasing contribution by partial volumina exposed to only small effective radiation intensities. In other words, a single chamber photoreactor having a predetermined depth d will put the UV radiation incident from the radiation source to optimum use only for media within a relatively narrowly limited range of UV transmissions.
The problem to be solved by the invention is to provide for a process and for an apparatus of the kind initially mentioned permitting optimum utilization of the incident radiation. Such a reactor should be designed, if possible, so as to provide for optimum utilization of the UV radiation over a wide range of UV Transmissions of the media to be irradiated or, respectively, at least within the variation in the UV transmission of the medium to be irradiated. Thereby, specifically in connection with water disinfection, should be ensured that the rate of flow of the medium in the irradiation chamber immediately adjacent to the radiation source is sufficient to prevent depositions to occur even under the action of high radiation intensities.
Said problem is solved by the characterizing features as stated in the claims. Advantageous designs and further developments of the invention are characterized by the features in the subclaims. Particularities thereof will be discussed in connection with the embodiments.
Embodiments of the apparatus according to the invention are shown in the drawings and will be explained and described in detail hereinbelow.